(290c) Model-Based Comparison of Novel Hollow-Fibre Sorbent Adsorber to Packed Bed Process for Dilute Gas Streams

Adsorption provides a relatively less energy intensive alternative to capture dilute streams from gaseous mixtures. In certain applications such as removal of CO₂ from air, recovery of valuable VOCs from exhaust streams, separating streams with almost identical thermodynamic properties (such as separation of ethylene/ethane); adsorption is often the only route to making the process commercially viable. Process improvement for the recovery of dilute gases has become an important area of research in the recent past. This may be attributed to the unprecedented rise in atmospheric concentration of CO₂ (>400 ppm for the first time in 4 million years^[1]) and, the implementation of the Maximum Achievable Control Technology (MACT) provisions of the Clean Air Act, which make the reporting and control of VOC emissions mandatory. Capture of dilute streams also finds applications in upgrading processes for methane, inevitably extracted as a co-stream from natural gas reservoirs.

Traditional processes for adsorption involve use of TSA, PSA or VSA in the packed bed configuration. To improve on the current process, a novel hollow fibre configuration for adsorbents was proposed by Lively et. al.using 13X as a â??proof of conceptâ? zeolite^[1]. The key advantage of this configuration is the substantial reduction in pressure drop relative to the packed beds. This is of primary importance when dealing with dilute streams where, additional pressure drop may involve significant capital investment. Additionally, the presence of the impermeable lumen layer allows the use of cooling agents without being limited by its adsorption properties, unlike the situation in a packed bed. This is a significant advantage for dilute streams, where TSA is a more economical process alternative compared to PSA. The fast heat transfer in the hollow fibre configuration shortens the cycle time, therefore increasing the throughput and reducing the quantity of adsorbent. One possible drawback of the hollow-fibre configuration maybe the parasitic heat load of the fibre material itself during the desorption step, which can be mitigated by heat integration.

This talk addresses the comparison of an adsorption-desorption cycle (TSA) of a dilute gas stream in both the previously mentioned configurations, packed bed and hollow fibre, using mathematical models. Coupled PDEs are obtained from heat and mass balance. The form of the equation is dictated by the system geometry. Mass transfer models appropriate for each configuration are also used, along with isotherm equations which are configuration independent. The commercial, dynamic process modelling software gPROMS is used to solve the system of equations. The models are used to compare the pressure drop of a dilute gas stream in both configurations, during adsorption. The separation performance is compared for the two configurations taking into account the pressure drop, which is dictated by the gas velocity. Finally, the parasitic heating load of the fibre material is estimated.

[1] 'Modeling of rapid temperature swing adsorption using hollow fiber sorbents', 2014, Rezaei, F, Subramanian, S, Kalyanaraman, J, Lively, R P, Kawajiri, Y, Realff, M J ,Chemical Engineering Science, 113, 62-76